Synapse formation, synapse stabilization and neurotrophic effects on ionic channles were studied using neurons and muscle cells in culture. Synapses were detected and investigated by electrophysiological recording. Single ionic conductances were studied with the extracellular patch clamp technique on cultured neurons and muscle cells and on bilayers formed at the tip of patch pipets. Neurons from chick spinal cord form stable cholinergic synapses with muscle cells in culture, eliciting changes that resemble maturation in vivo. In vitro innervation blocked an apamin-sensitive Ca2+ dependent K+ conductance, an effect that can be elicited in the absence of innervation by a low molecular weight fraction (less than 4,000 D) obtained from chick spinal cord conditioned medium but not from a spinal cord extract. Calcium channel blockers impair the ability of dissociated neurons to form synapses with muscle cells in culture, and inhibit neurite extension. The degree of inhibition depends on the age in ovo and is more marked in neurons dissociated from older embryos. Conditions were established for maintaining fetal dorsal root ganglion neurons in primary culture for up to two months. Electrophysiological recording from a large number of neurons from normal tissues generated a control data base of passive and active electrical membrane properties. Electric field generated during neuronal activity can promote electrophoretic migration of charge substances within the cell of cell membranes, sufficient to enhance synapse stabilization within dendritic spines.